Nowadays, lithium ion secondary batteries are used in a wide variety of applications ranging from power sources for relatively small electronic devices such as mobile phones, digital cameras, and personal computers to those for large-sized machinery such as electric automobiles and power tools. The diversified demand for performance of the lithium ion secondary batteries has been increasing, and adaptation for various fields is under energetic study. Lithium ion secondary batteries for the in-vehicle applications among others are required to achieve performance on the assumption that they would be used in harsh environments.
In particular, there has been demand for improvement in power density and energy density in view of high performance, and also demand for suppression of capacity degradation at high temperatures and low temperatures and improvement in cycle life and safety in view of high reliability.
Various attempts have been made to solve the above-mentioned problems, leading to improvements. As means for improvement, there have been examined optimization of an active material used for a cathode material or an anode material, combinations and blend ratios of salts and solvents that constitute an electrolyte solution, combinations and blend ratios of additives for improving characteristics, and the like.
Degradation and deterioration of a non-aqueous electrolyte solution for a lithium ion secondary battery can be suppressed by appropriately selecting combinations and blend ratios of the above-mentioned constituent members. Such a suppressing effect is a factor in significantly improving the characteristics of the lithium ion secondary battery such as performance and reliability.
Under these circumstances, Patent Literature 1 discloses that when a non-aqueous electrolyte solution containing as an additive at least one selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate is used, the additive reacts with lithium used as an electrode to form a good-quality coating on the surface of the cathode and the surface of the anode, and that these coatings suppress the contact between active materials in a charging state and an organic solvent to thereby suppress the degradation of the non-aqueous electrolyte solution due to the contact between the active materials and the electrolyte solution, thus improving the storage characteristics of the battery.
Some methods for producing a difluorophosphate such as lithium difluorophosphate described above have been examined. For example, Patent Literature 2 discloses a method involving a reaction between a borate and lithium hexafluorophosphate. Patent Literature 3 discloses a method including adding a halide to lithium hexafluorophosphate, and then reacting the mixture with water in a non-aqueous solvent. Patent Literature 4 discloses a method involving a reaction between a phosphorus oxoacid, a hexafluorophosphate, and an alkali salt in the presence of hydrogen fluoride to produce a difluorophosphate. Patent Literature 5 discloses a method including reacting phosphorous oxychloride with lithium carbonate to synthesize lithium dichlorophosphate and then bringing hydrogen fluoride into contact therewith to produce lithium difluorophosphate. On the other hand, purification methods have also been examined, and, for example, Patent Literature 6 describes a method including bringing hydrogen fluoride into contact with a difluorophosphate. However, in this method, a hexafluorophosphate may be produced as a by-product and degraded in the system, and this degradation causes coloration of crystals of the difluorophosphate. Moreover, the effect of reducing a free acid component (acidic impurity) and the effect of reducing an insoluble residue are small in this method, and it is thus difficult to obtain a difluorophosphate having a high purity. Therefore, it is difficult to say that this method is efficient.
Moreover, there also are needs for methods for efficiently producing and purifying, on an industrial scale, difluorophosphates other than a lithium salt, such as sodium difluorophosphate, potassium difluorophosphate, and ammonium difluorophosphate.